Air is fed into a combustion chamber of an engine through an intake passage in the intake manifold (hereinafter referred to as the “intake manifold passage”) and an intake passage in the cylinder head (hereinafter referred to as the “intake port”).
Since the intake manifold and the cylinder head are heated by heat transferred from the combustion chamber, suction air tends to be heated by heat from the inner surfaces of the intake manifold passage and the intake port.
Especially in a high-compression-ratio engine, an increased suction air temperature tends to cause knocking more frequently than in a low-compression-ratio engine. To prevent knocking, it is necessary to e.g., retard the ignition timing. Since retarding the ignition timing worsens fuel economy, it is desired to minimize the temperature rise of the suction air.
To minimize the temperature rise of the suction air, the below-identified Patent Document 1 proposes a heat insulating member for suction air which comprises a material low in thermal conductivity, such as resin, and closely adhered to the inner surface of the intake port, which is made of metal.